A monochromator for examining a narrow bandwidth or range of a light spectrum is a spectroscopic apparatus having a stop-down which is a narrow bandwidth of a spectrum generated by a grid or grating. Monochromators are known in which a grating is illuminated by light passing through the entry slit and is rotated by a grating drive. A stationary exit slit is disposed within the angular range of the generated spectrum. The swiveled cutoff filter is disposed in the beam path between the grating and the exit slit for filtering out light of the orders other than the order to be examined.
When the grating is rotated, the spectrum passes by the exit slit such that certain light frequencies can be examined depending on the angular position of the grating. An entire spectrum can be scanned if the grating is swiveled continuously.
Gratings are used because of their high resolution but gratings have the disadvantage that they always generate spectrums of several orders. The 0 order cannot be used because it does not allow a spectral analysis. The lower orders have increasing light intensities as well as an increasing dispersion (spectral resolution). The 1st order is used most of the time for reasons of intensity. In this, one disturbing factor can be seen in the fact that the different orders overlap one another. The light of long wavelength of the 2nd order is situated at the location of the light of short wavelength of the 1st order. For this reason cutoff filters are required to allow only light above a cutoff wavelength to pass and consequently are suitable to filter out the light of the 2nd order when examining the light of the 1st order. However, this is possible only within a certain spectral range. If the arrangement operates with a cutoff filter that has a cutoff wavelength of, for example, 800 nanometers (nm), and it is desired to examine light of shorter wavelengths, the cutoff filter must be swiveled out of the beam path and replaced by a filter of shorter cutoff wavelength.
In known monochromators of this type, the cutoff filters consequently are arranged in such a way that they are swiveled in and out of the beam bath if the grating exceeds or falls short of certain rotation angles.
One construction of this type is known from European Patent No. 0,015,6070 B1 (FIG. 2). The cutoff filter according to this publication is moved incrementally by means of a continuous cam disk with a stepped curve. The incremental movement of the cutoff filters interferes with the adjustment of the monochromator, namely because it impairs a highly precise, continuous adjustability.
Another construction of this type is known from German Utility Model No. 8,134,940. In this, a cutoff filter is adjusted continuously via the drive of the grating which is an adjusting knob. However, the adjustment of the cutoff filter is carried out in linear relationship to the adjusting knob, while the grating moves in a sinusoidal fashion. The angular positions of the grating and the filter only correspond at one wavelength. Consequently, it is not adaptable for using filters with several cutoff ranges. In addition, a displacement of the cutoff filter to a different spectral range is very difficult.
A general object of this invention is to provide a monochromator of the type discussed above but which allows arbitrary spectral ranges to be scanned continuously with high accuracy while filtering out interfering orders.